Chromites and platinum group metals occur in potential association in specific geological environments such as stratified and layered mafic to ultramafic magmatic complexes that have intruded continental rocks. The term “platinum group metals”, usually referring to the metals platinum, palladium, iridium, ruthenium, rhodium, osmium, is referred to herein as “PGM”. PGM rich chromitites are extremely interesting ores because of their double economic values as: 1) a source of chrome for ferrochromium production, a master ferro-alloy for the stainless steel industry; and 2) a source of metals for the PGM industry.
Presently, there are only a few large mining producers that operate metallurgical facilities capable of extracting PGM from chromitites, and these producers are all based in the Republic of South Africa (RSA). The PGM extraction process of RSA, according to the review of known processes presented by Vermaak 1995, is based on: 1) the production of flotation concentrates which are then submitted to 2) smelting, 3) converting, 4) base metal extraction and 5) PGM purification. This PGM extraction process requires the production of a flotation concentrate and the development of a large metallurgical infrastructure. When considering the large variety of mineralogical composition of the phases carrying the PGM in chromites deposits, their grain-sizes distribution and the PGM concentration, the prospect of efficiently producing a flotation concentrate from chromite ores is often very limited. In addition, building a metallurgical infrastructure based on smelter and on associated technologies is costly and not economically adapted to the extraction of PGM from small and medium scales deposits. There is a need for an improved method of recovering PGMs.
Bergeron, Laflèche, PCT co-pending application no PCT/CA2004/000165 filed on Feb. 6, 2004 discloses a method for carbochlorinating chromites. In that process, a chromite product mixed with NaCl is contacted with chlorine and carbon monoxide in a reactor maintained at temperatures of 157° C. to 750° C. to convert the iron oxide in the ore into gaseous iron chloride which is removed and condensed. The chemical reaction at the heart of the process is: FeO.Cr2O3+1.5Cl2(g)+CO(g) Cr2O3+FeCl3(g)+CO2(g). The solid material resulting from the process shows: 1) a large increase in its chromium to iron ratio; and 2) a residual enrichment in bulk Cr2O3 content. Both effects boost the ore's trading value.
Three broad techniques in the field of chlorine metallurgy can be identified. Broadly, 1) the carbochlorination technique involves using gaseous chlorine in the presence of a reductant such as carbon monoxide, usually chosen for process development, or coke. 2) The chlorination technique involves the use of chlorine without the addition of a reductant agent. 3) The third technique, chlorination in the presence of a salt melt, involves the addition of a large quantity of salt so as to form a molten bath of salt, with or without the generation of gaseous chorine. The carbochlorination, chlorination and chlorination in the presence of a salt melt techniques differ in the chemical reactions that are involved in each of them.
Carbochlorination
The effect of carbochlorination on PGM values contained in spent automotive catalyst is described in the prior art. Rivarola et al., 1981, Lat. am. j. chem. eng. appl. chem., 11, 47-52, describe the volatilization of platinum from Al2O3 spent catalyst by a chlorine-carbon monoxide mixture. The recovery of platinum, as a volatilized phase, yielded extraction closed to 100%. The influence of temperature, time, and gas flow rates were investigated. Kim et al., 2000, Ind. Eng. Chem. Res., 39, 1185-1192, also describe the carbochlorination of spent automotive catalyst to extract the platinum and rhodium values. After optimization of time, temperature, gas flow rates, partial pressures of chlorine and carbon monoxide, recovery of 95% of platinum and 92% of rhodium were obtained.
U.S. Pat. No. 5,102,632 issued to Allen et al., 1992, relates to a method of recovering platinum, palladium and rhodium dispersed on ceramic support structures. The process involves two steps. In a first step a reducing chlorination is carried out during which the palladium and platinum are volatilized as chlorides. In a second step only chlorine is used to volatilize rhodium trichloride.
Although certain effects of carbochlorination on PGM for other types of ores, concentrates, metallurgical products and materials were known, the effect of carbochlorination on PGM values contained in chromites was never disclosed. The present invention teaches the effect of a formation of FeCl3 on the vapor transportation of PGM and teaches a new process for the recovery of PGM from chromite products and other concentrates.
Chlorination
Extraction of PGM by chlorination from sulphides flotation concentrates was investigated by Cooper and Watson, as early as 1929, J. Chem. Metal. Min. Soc. S. A., 220-230. According to their method, a sulphides flotation concentrate is roasted, mixed with 15-20% of NaCl and chlorinated at 550° C. for six hours. After the chlorination step, the solid is leached with concentrated HCl, PGM are cemented with zinc dust and the solution is filtered to isolate a PGM concentrate.
South African Patent 96-2382 issued to Lalancette and Bergeron, 1996, describes the chlorination of chromites ore for the extraction of PGM. The method described involves mixing the ore with NaCl 10% wt/wt, dry chlorination of the mixture between 350° C. to 800° C. with gaseous chlorine, dissolution of PGM in concentrated hydrochloric acid solution, filtering and recovering the PGM from the solution. PGM recoveries are reported to be in the order of 95 to 100%.
Canadian Patent application no. 2,303,046 in the name of Prior, 1999, teaches the extraction of PGM from a material derived from the smelting of sulphides concentrates rich in base metals. The material is subjected to three gaseous treatments, an oxidizing treatment, a reducing treatment and a chlorination treatment at elevated temperature. After the gaseous treatments, the material is leached with HCl or aqua regia and the precious metals recovered by a chromatographic procedure.
Canadian Patent application no. 2,314,581 in the name of Craig and Grant, 2000, describes a method for the removal of base metals, especially the amphoteric elements present in metallurgical concentrates containing 60 wt % and more of precious metals. The presence of the base metals in the precious metals concentrates is considered to be detrimental to the down stream refining steps. The method described comprises the following steps: a) a high temperature treatment of the concentrate with gaseous HCl; b) a treatment of the residue, if desirable, with chlorine gas, c) a high temperature treatment of the residue with oxygen, d) a high temperature treatment of the residue with hydrogen. This procedure minimizes losses of precious metals during the removal of the amphoteric elements.
Salt Melt Chlorination
U.S. Pat. No. 5,074,910 issued to Dubrovsky, 1990, teaches the recovery of precious metals from base metals sulphide ores by chlorination in a molten salt bath in the presence of chlorine gas. The feed is pressed into pellets with addition 50% wt/wt of salt, feeded to a reactor and contacted with chorine gas at a temperature producing a molten salt bath. After the complete conversion of the precious metals into chlorides, the precious metals are then recovered from the melt by a suitable means.
U.S. Pat. No. 5,238,662 issued to Dubrovsky, 1993 describes the recovery of precious metals contained in a matte obtained from the smelting of sulphide concentrates rich in base metals. The matte is contacted with gaseous chlorine in a molten salt bath to effectively convert the PGM into their chlorides forms. A further selective dissolution technique for PGM involving multiple dissolution stages is also presented.
As indicated earlier, processes based on chlorination and salt melt chlorination for PGM recovery from ore, minerals and metallurgical concentrates involve chemical reactions that differ from those involved in carbochlorination. Furthermore, although carbochlorination was performed on PGM contained in spent catalyst, prior to the present invention thus, no data existed on the behaviour of PGMs during the carbochlorination of PGM rich chromites products and other concentrates.
Hence, there is a need to develop a process that can extract PGM from ores and concentrates including chromites. This process would desirably be adaptable to a situation where a chromite is subjected to an enrichment process as described in co-pending no PCT/CA2004/000165 by Bergeron and Laflèche by which the iron is extracted as gaseous FeCl3. This treatment could desirably be designed so that it could be performed simultaneously to the enrichment of chromites.